As it is well known, the classical modulation PSM consists in driving two half-bridges of a Full-Bridge converter with fixed duty-cycle, of value close to 50% so as to have symmetrical losses by conduction between the four switches of a primary winding. In any case the duty-cycles of the two half-bridges must be as close as possible since, in case of the use of a Current-Doubler with a secondary winding, an asymmetry thereof would reflect in a difference between the currents of the filter inductances.
By turning on the two diagonals of the two half-bridges alternatively, for each switch period four main operation states of the Full-Bridge converter are obtained. With reference to FIG. 1, in the two states wherein a high-side switch (Q1 or Q3) of a half-bridge (S1 or S2) and a corresponding low-side switch (respectively Q4 or Q2) of the other half-bridge (S2 or S1) of the Full-Bridge converter 10 are on, i.e. the turn-on of one of the two diagonals causes energy transfer towards an output terminal OUT.
The other two states occur when either the two high-side switches (Q1 and Q3) or the two low-side switches (Q2 and Q4) are on. In this situation there is no energy transfer towards the output terminal and the current of the primary recirculates in the upper or lower mesh of the Full Bridge converter 1.
By modulating the ratio between the duration of the transfer periods and the duration of the recirculation periods, a variation of the output voltage is obtained. This is exactly the mechanism exploited in the converters realized according to the prior art to obtain the output voltage regulation.
In a system with analog control the output voltage, as a regulated variable, is fed back at the input of an error amplifier, so as to obtain a control variable which allows shifting in phase the two waves PWM controlling the two half-bridges S1 and S2.
A classical digital controller converts the output voltage into a digital magnitude to obtain, downstream of a digital regulator, a quantized phase shift between the two waves PWM.
As it is known, the precision required by the application determines the resolution of an input connected A/D converter.
So as not to have evident limit cycles, the digital modulator PWM should exhibit a higher resolution than that previously calculated. In practice the number of discrete phase shift values the A/D converter is supplied with is determined.
The solutions adopted up to now are based on current-mode control. For such known solutions it occurs that the analog front-end appears to be inadequate due to circuit complexity. The voltage control systems are instead simpler, but they cannot provide a current-sharing control.
In fact the only limitation the electric network realizing the Current-Doubler imposes is that the sum of the currents in the two filter inductances is equal to the load current. It is evident that any asymmetry of the system would lead to having different currents.
Thus, up to now, the Current-Doubler has been used in systems with current control. If only a control with peak current is considered (but this is also valid for controls with mean current) this implies the need of reading the absolute value of two currents by means of a trans-impedance circuit, and then of comparing them with an analog voltage reference suitably generated by the controller.
To such purpose, if the controller is of the digital type, a D/A converter is used.